For long range communications utilizing low frequency (LF) and very low frequency (VLF) transmissions, both for point-to-point and relay communications, one of two basic radiated field propagation modes may be used, the TM mode with a dominant vertical electric field component, and the TE mode with a dominant vertical magnetic field component.
Based on theoretical analysis and measured data on VLF propagation characteristics, the TE mode is particularly well suited to low frequency transmission because of its lower average attenuation rate of a fraction of a db per one thousand kilometers over the entire frequency range as compared to two to three db per thousand kilometers for the TM mode. Furthermore, the TM mode attenuation is highly variable, being affected by the effective ionospheric height, among others. The attenuation rate for the TE mode is fairly stable for these conditions.
In the case where the ionosphere is depressed by a high altitude nuclear detonation, the degradation of transmission in the TM mode is more severe than the worst case of a non-nuclear environment, while the change in the attenuation rate for the TM mode appears to be only nominal.
However, transmission of LF and VLF frequencies in the TE mode is difficult to achieve since the field radiated from a source close to the earth's surface consists predominantly of TM modes regardless of the excitation source. More specifically, in the "wave guide" defined by the earth and ionosphere, both vertical and horizontal dipole antennas, located close to the earth's surface, radiate essentially TM modes at any reasonable distance away from the source. When the antenna is a loop antenna with its plane parallel to the earth's surface, the image of the antenna virtually cancels the field radiated from it. Such a transmission, therefore, is extremely inefficient.
From the above, it can be seen that the TE mode is desirable for its transmission characteristics, and that an improved means for transmitting and receiving in this mode is required for reliable communication, especially during a national emergency.